Experiments in digestion

Communicating the nature of science through historical perspective is a tool to illustrate a particular development in science thought.

How does the digestive system work?

Primitive man had little knowledge of digestion. Only much later in the scientific era, man began to search for answers for why digestion was necessary and why nature provided a gastrointestinal tract and digestive juices.

The two articles, “Man’s curiosity about food digestion: An historical overview” and “A short history of early digestion trials”, emphasise the journey through history of food digestion. These articles start with the early belief systems about digestion and how the scientists proved their hypothesis through experimenting. This will enable students to appreciate how the concepts of science evolve with time.

Is digestion similar to cooking... Or is it Mechanical... Or Fermentation...?- Exploring early belief systems:

Aristotle (384- 322 BC) proposed the role of saliva in digestion and believed that the stomach cooked ingested food, and the intestine absorbed the product.

Later, scientists proved that digestion was a mechanical process by observing stomach movements.

Van Helmont (1572–1644) described stomach digestion as fermentation.

Experiment 1 - Is it mechanical?

The bird buzzard (a large hawk like bird of prey), is capable of ejecting indigestible substances. A French scientist, Rene Antoine de Reaumur, fed this bird, a variety of foods, such as meat, vegetables and grains, encased in small metal cylinders with both ends covered with perforated metal gauze (Why do you think he preferred a metallic cylinder?). On researching the bird’s expelled food, he noted partially dissolved food, but the metallic cylinder remains intact. He inferred that digestion is a non-mechanical process.

Experiment 2 - If it is non-mechanical, what else it could be?

Exp.01- He collected pure gastric juice with encased sponges that he fed to the buzzard, and when these were recovered, he found the gastric juice was salt and sour in taste. Through this observation, can you guess what could be the nature of gastric juice?

Exp.02- He then poured it onto a food substance

He observed that food was softened but not dissolved. (Why do you think, the food was not digested fully?). Because he failed to keep the mixture at body temperature, the result was reduced efficiency in the action of gastric juice.

He inferred that digestion is due to the solvent action of gastric juices.

Experiment 3 - Is the digestion similar in human?

To determine whether observations made about animals (experiment done by Reaumur) were applicable to humans.

Spallanzani, an Italian biologist, swallowed food, encased in perforated tubes, and recovered them through the anus. In addition, he kept food with gastric fluids at body temperature. What would have happened to the swallowed food and the food kept with gastric fluids at body temperature?

He observed that swallowed food was digested well (similar to Reaumur’s experiment with animals) and food kept with gastric juices was readily dissolved (overcoming the failure of Reaumur’s experiement).

Experiment 4 – what is in Gosse’s vomit?

In 1760, Mr. Gosse who had developed the ability to vomit by swallowing air, started observations on digestion. He would eat essentially the same food upon "diverse occasions" and after various lengths of time would vomit it up and examine the results.

Observation:

Half an hour- not much change, foods retained change

1 hour- pulpy, well mixed with gastric juice

2 hours- very pulpy and only half of the food left

This result led him to infer that different foods need different times for digestion.

Experiment 5- The miracle wound

A bullet hit a soldier named St Martin, although the wound healed, the hole in the stomach remained open. Dr. Beaumont, who treated him, was interested to find out if the stomach contained digestive juices all the time, even when no food was present.

Exp. 01: First, he asked St Martin to eat nothing for a few hours. Then he looked inside the stomach. When he looked inside the stomach what could he probably had observed? He found that stomach contained saliva, which St Martin had swallowed, and some mucus from the stomach wall.

Exp. 02: Beaumont put some bread crumbs into the stomach. After 1 hour, what he could have probably observed? He saw digestive juices start to collect on the wall of the stomach.

Inference:

Digestive juices are not present in the stomach all the time. There should be contact between the food and the mucosa for the secretion of digestive juices.

What happens to the food in the stomach?

Exp. 01: Beaumont attached different foods like cooked and raw meat, bread and cabbage onto silk strings and pushed them through the hole.

Observation-

1 hour later: Half the cabbage and bread had broken up but the meat remained the same.

2 hours later: cooked meat had started to break down.

Exp.02:

Beaumont wanted to find out what happened to the food after St Martin had eaten it. He gave St Martin a meal of fish, potatoes, and bread.

Observation-

Half an hour: He could still identify pieces of fish and potato.

One hour: Pieces of potato could still be seen but most of the fish had broken up.

One and half-hours: All the pieces of food had broken up.

2 hours- The stomach was empty.

Inference: Different digestion time is needed for different food and the time taken for the entire food to be digested completely was found. It was also concluded that digestion starts in the mouth due to salivary action.